In a radio communication system in recent years, scheduling of data transmission on a downlink such as HSDPA (High Speed Downlink Packet Access) is attracting attention on one hand and scheduling of data transmission on an uplink is under study on the other.
During scheduling on an uplink, a base station apparatus sequentially assigns data transmission timings to communication terminal apparatuses within the own cell and the communication terminal apparatuses transmit their signals at the respective assigned transmission timings.
More specifically, for example, the base station apparatus estimates the channel quality of the downlink up to the respective communication terminal apparatuses from transmit power when transmitting signals to the respective communication terminal apparatuses within the own cell. Then, uplink scheduling is realized by assigning transmission timings on the uplink to the communication terminal apparatuses in descending order of channel quality (e.g., see Unexamined Japanese Patent Publication No. 2002-290327).
However, the above described uplink scheduling has a problem that even when there is a communication terminal apparatus in the process of soft handover within the own cell, the base station apparatus performs uplink scheduling regardless and independently of the base station apparatus at the handover destination of the communication terminal apparatus in the process of soft handover. That is, when a communication terminal apparatus has established communications with a plurality of base station apparatuses to realize soft handover, if the communication terminal apparatus sends a signal at a transmission timing assigned by one base station apparatus, the signal also arrives at other base station apparatuses which are communicating with other communication terminal apparatuses and causes interference with their communications.
This problem will be explained more specifically with reference to FIG. 1.
In FIG. 1, communication terminal apparatuses (MS) #3 and MS#4 are communicating with a base station apparatus (BTS) #1 and MS#5 and MS#6 are communicating with a BTS#2. Furthermore, the MS#1 and MS#2 are moving from the cell of the BTS#1 to the cell of the BTS#2 (or from the cell of the BTS#2 to the cell of the BTS#1) and are in the process of soft handover which allows the MS#1 and MS#2 to communicate with both of the base station apparatuses BTS#1 and BTS#2.
Under such circumstances, for example, the BTS#1 performs uplink scheduling and a transmission timing is assigned to the MS#1 which is in the process of soft handover. At this time, the MS#1 sends a signal at the assigned transmission timing and this signal reaches not only the BTS#1 but also the BTS#2.
On the other hand, the BTS#2 is also autonomously performing uplink scheduling and if a transmission timing has been assigned, for example, to the MS#6 when the signal from the MS#1 arrives, the signals from the MS#1 and MS#6 arrive at the BTS#2 simultaneously and the signal from the MS#1 to which no transmission timing is assigned constitutes an interference component in the received signal of the BTS#2.